Of the two it has been Russia on the international stage that has thwarted US and Western efforts to halt 18 months of bloodshed in Syria. But on the ground it is Iran that appears to be more active in efforts to prop up what some regional experts and defectors from Mr. Assad’s side say is a crumbling regime.

Speaking to reporters Tuesday, the Pentagon chief said Iran’s “growing presence” is “adding to the killing that’s going on in Syria.” He said the US has evidence that Iran is attempting to train and deploy a new militia to fight on Assad’s behalf.

The Assad regime has long had at its disposal the Shabiha, a militia believed to be responsible for many of the massacres that have taken place in Sunni villages.

Mr. Panetta’s comments followed claims by a number of sources, including Iranian exile groups, that the Iranian “pilgrims” that Tehran said were kidnapped by Syrian rebels and held hostage were actually members of Iran’s Islamic Revolutionary Guard Corps (IRGC), who were in Syria to support Assad.

According to the Mojahedin-e Khalq (MEK), an exile group opposed to Iran’s clerical regime, the 48 detained Iranians were part of a larger contingent of 150 members of the elite IRGC dispatched to Damascus to assist Assad.

The MEK says the IRGC deployment is just one sign of Iran’s determination to keep ally Assad in power – a policy the Iranian regime has not been shy about publicizing. During a recent trip to Damascus, the national security representative of supreme leader Ali Khamenei, Saeed Jalili, went on Syrian state television to declare Syria’s war “a conflict between the axis of the resistance and its enemies in the region and the world.”

Those stark terms prompted some regional analysts to characterize Tehran as more invested in Assad – and willing to go farther on his behalf – than Russia.

Moscow has protected Assad from international intervention in Syria through the United Nations, but insists it is opposed to any outside interference in what it says is an internal Syrian conflict. Many experts believe Russia would drop its support for Assad if it deemed his hold on power to be unsalvageable, and if it felt assured that its interests would not suffer in a post-Assad Syria.

The MEK says the detention of the 48 Iranians led IRGC commanders in Damascus to order the remaining members of the 150-strong contingent to return to Iran. But it says the IRGC’s policy remains to beef up its presence in Syria, and to that end the Iranian exiles, who claim to have informants with the regime in Tehran, say the IRGC commander in Lebanon has been transferred to Damascus.

Since 1997 the MEK has been on the State Department’s list of terrorist organizations for violent acts it carried out primarily in Iran during the Iran-Iraq war, but it is fighting in court to be de-listed. It claims to have renounced violence and says it has provided the US government with intelligence on Iran’s nuclear program and global activities.

Secretary Clinton highlighted the US concerns about Iran’s involvement in the Syrian conflict when she met Saturday in Istanbul with Turkish Foreign Minister Ahmet Davutoglu. She said that new sanctions announced the day before by the Treasury Department were part of an effort “to expose and disrupt the links between Iran, Hezbollah, and Syria that prolong the life of the Assad regime.”

Among other things, the new sanctions target the Syrian state oil company for trading with Iran, and ties Hezbollah, already designated a terrorist organization by the US, to the Assad regime’s violence.